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Physiology of the Midbrain

Mesencephalon

For medical students2 min readUpdated 2026-10-10

The midbrain is a critical region of the brainstem responsible for postural regulation, muscle tone maintenance, and complex motor acts. Additionally, it serves as a relay for key ascending and descending pathways connecting higher centers with the spinal cord.

Mesencephalic animalAn experimental preparation with a brain transection at the level of the superior colliculi.

NeurotransmitterNeurons of the substantia nigra synthesize dopamine.

RigidityDestruction of the red nuclei results in severe extensor muscle spasm.

PathwaysConduction functions are mediated through the cerebral peduncles (*crura cerebri*).

Key Midbrain Structures and Functions

The midbrain contains several critical nuclear groups, each performing specialized tasks:

Substantia nigra. Composed of dopaminergic neurons. It regulates emotional behavior and fine motor control (such as finger movements). It also coordinates sequential complex motor acts like chewing and swallowing.
Periaqueductal gray (Periaqueductal gray). Serves a unique receptor function regarding cerebrospinal fluid (CSF). Its primary role is maintaining homeostasis and the constancy of the internal environment.
Noradrenergic system. Represented by neurons of the locus coeruleus (Locus coeruleus) and adjacent ventrolateral reticular formation areas. Axons from these cells project broadly to the brainstem, cerebellum, hypothalamus, hippocampus, amygdala, and the cingulate and entorhinal cortex.

Red Nuclei and Muscle Tone

Unlike a spinal animal, a so-called "mesencephalic animal" (whose forebrain is separated from the midbrain by a transection at the level of the superior colliculi) can independently maintain body posture and preserve adequate muscle tone. Such a preparation does not exhibit decerebrate rigidity because the midbrain and its red nuclei (Nucleus ruber) remain intact.

The red nuclei exert a continuous inhibitory influence on the vestibular Deiters' nuclei located in the medulla oblongata. Deiters' nuclei, in turn, receive excitatory afferentation from muscle proprioceptors and otolith organs via the vestibulospinal tract.

If the red nuclei are destroyed, this inhibition of Deiters' nuclei ceases. Consequently, activity in extensor muscles increases selectively and sharply at the hindbrain level, leading to decerebrate rigidity. This rigidity is reflex-mediated, sustained by a continuous stream of impulses from proprioceptors and the vestibular apparatus. This is proven by the fact that sectioning the vestibular nerves or the dorsal roots of the spinal cord completely abolishes the muscle spasm.

Conduction and Reflex Functions

Midbrain functions are broadly divided into intrinsic (reflexive) and transit (conduction) categories.

1. Conduction function. Implemented via the structures of the cerebral peduncles, which contain major tracts:

Ascending (afferent) pathways: spinothalamic tract and medial lemniscus.
Descending (efferent) pathways: pyramidal and corticopontine tracts.

2. Reflex function. The midbrain regulates posture via tonic reflexes, classified by R. Magnus and A. de Kleijn. These reflexes are divided into:

Static reflexes: include postural reflexes and righting reflexes, which restore normal posture when disrupted.
Statokinetic reflexes: provide compensatory reactions during body acceleration in space.

Mnemonic

"Red brake, black motor": Red nuclei brake extensors (preventing rigidity), while the Substantia nigra initiates fine motor control and complex acts.

Frequently asked questions

Which cranial nerves originate from midbrain nuclei?

The third and fourth cranial nerve pairs originate from midbrain nuclei.

Oculomotor nerve (n. oculomotorius, cranial nerve III) — its motor and autonomic parasympathetic nuclei are located in the superior colliculi of the midbrain tectum.
Trochlear nerve (n. trochlearis, cranial nerve IV) — its sole motor nucleus is localized in the inferior colliculi of the midbrain tectum.

What functions are subserved by the superior and inferior colliculi of the corpora quadrigemina?

The superior and inferior colliculi act as primary sensory processing centers, mediating subconscious orienting reflexes.

Superior colliculi — primary visual centers processing visual information.
Inferior colliculi — primary auditory centers processing sound information.

Collicular neurons mediate visual and auditory startle responses via the tectospinal tract. Components of these reactions include turning the head to listen, moving the eyeballs, adjusting pupil size, and rotating pinnae in animals.

Which structures form the roof of the midbrain (tectum mesencephali)?

The roof of the midbrain is formed by the tectal plate (corpora quadrigemina), comprising four elevations on the dorsal surface of the brain. Its structures include:

Superior colliculi — two larger elevations.
Inferior colliculi — two smaller elevations.

Topographically, the midbrain roof is bounded by an imaginary horizontal plane passing through the cerebral aqueduct. The pineal gland is also situated near the corpora quadrigemina.

What is the difference between a mesencephalic animal and a spinal animal?

A mesencephalic animal retains the ability to maintain normal muscle tone and spatial body posture, whereas a spinal animal loses these functions.

What is the mechanism underlying decerebrate rigidity?

It occurs when the red nuclei of the midbrain are destroyed. Normally, they inhibit Deiters' nuclei; without this inhibition, extensor muscles become hyperactivated.

How can the reflex nature of decerebrate rigidity be demonstrated?

Transecting the dorsal roots of the spinal cord or the vestibular nerves interrupts afferent input, causing the rigidity to disappear.

What is the function of the periaqueductal gray matter?

It performs a receptor function regarding CSF and helps maintain internal homeostasis.

Go deeper

Mechanisms of static and statokinetic Magnus and de Kleijn reflexes
Connections and functions of dopaminergic neurons in the substantia nigra
Organization of the vestibulospinal tract and Deiters' nucleus
Projections of locus coeruleus pathways to the cortex and limbic system